重建二叉树

import java.util.ArrayDeque;

import com.sun.swing.internal.plaf.basic.resources.basic;


public class Test {

	//重建二叉树
	public static TreeNode reConstructBinaryTree(int [] pre,int [] in) {
		return reConstructBinaryTree(pre, 0, pre.length - 1, in, 0, in.length - 1);
	}

	private static TreeNode reConstructBinaryTree(int [] pre,int startPre,int endPre,int [] in,int startIn,int endIn) {
		
		//递归结束的条件
		if(startPre > endPre || startIn > endIn) return null;
		
		//从前序遍历中取出第一个元素，就是这棵树的根节点了(因为前序遍历是先访问根元素)
		TreeNode root = new TreeNode(pre[startPre]);

		//从中序遍历中找到  上一步的根节点，这样就可以把一棵树分为两部分了
		for(int j = startIn; j <= endPre; j++) {
			//在中序遍历中找到该元素的
			if(pre[startPre] == in[j]) {
				//第三个参数: (j - startIn)->代表左子树的节点个数，
				//这里递归调用会返回一个节点 并赋值回给 root.left
				root.left = reConstructBinaryTree(pre, startPre + 1, startPre + (j - startIn), in, startIn, startIn + j - 1);
				//第二个参数: (j - startIn)->表示左子树的节点个数，(startPre + 1)->表示偏移量
				root.right = reConstructBinaryTree(pre, (j - startIn) + (startPre + 1), endPre, in, j + 1, endIn);
				break;
			}
		}
		return root;
	}
	
	//按层遍历
	public static void print(TreeNode node) {
		if(node == null) return;

		ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();
		queue.offer(node);
		while(!queue.isEmpty()) {
			TreeNode poll = queue.poll();
			System.out.print(poll.val + ",");
			if(poll.left != null) queue.offer(poll.left);
			if(poll.right != null) queue.offer(poll.right);
		}
	}
	
	
	public static void main(String[] args) {
		int[] pre = {1,2,4,7,3,5,6,8};
		int[] in = {4,7,2,1,5,3,8,6};
		TreeNode root = reConstructBinaryTree(pre, in);
		print(root);
		
	}

}

class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;
	TreeNode(int x) { val = x; }
}